Please give additional information for me to assist you better:
Since when is the lump present?
Is it gradually increasing in size?
Pain or tenderness on pressure?
Is it on the back side of knee or at other places.
Does it reduce on bending the knee?
Any pain or discomfort during movements of the knee?

Please give more details and photos if possible as this help.
Such a large painless lump below the knee which does not reduce can be due to the following reasons:
Bursitis meaning inflammation with or without accumulation of fluid: This cannot be moved with skin.
Angio-neurotic edema under the skin: this is not attached to deep structures and hence can be moved with the skin.
This can be diagnosed by the following:
Clinical evaluation and examination by your Doctor or an Orthopedic Surgeon.
Ultrasonography/ MRI helps further and done if needed.
Until then give rest to the knee.

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